This invention generally relates to a method of preparing epitaxial films for use as infrared detectors, lenses, and light emission devices, and more particularly, to lead chalcogenide epitaxial films suitable for electro-optical applications. Additionally, this invention relates to a simplified method of controlling the composition or energy gap of the semiconductor material during epitaxial growth.
Thin-films of lead salt alloys have been investigated intensively recently with particular attention to their photovoltaic properties. Special attention has been paid to their possible use as detectors of infrared radiation.
The exploration of vacuum deposition techniques is quite recent and, for the benefit of those who may not be familiar with the pioneering efforts in this art, the following brief bibliography is made of record.
Preparation of Single-Crystal Films of PbS, by R. B. Schoolar and J. N. Zemel, Journal of Applied Physics, Vol. 35, No. 6, (June, 1964), pp. 1848 to 1851.
Epitaxial Lead-Containing Photoconductive Materials, by R. B. Schoolar, H. R. Riedl, and J. P. Davis, U.S. Pat. No. 3,574,140, (April 1971).
Method of preparation of Lead Sulfide PN Junction Diodes, by R. B. Schoolar, U.S. Pat. No. 3,716,424, (February 1973).
Method of Varying the Carrier Concentration of Lead-Tin Sulfide Epitaxial Films, by R. B. Schoolar, U.S. Pat. No. 3,793,070, (February 1974).
Photoconductive PbSe Epitaxial Films, by R. B. Schoolar and R. J. Lowney, Journal of Vacuum Science Technology, Vol. 8, No. 1, (1971).
Surface Charge Transport In PbS.sub.x Se.sub.1-x And Pb.sub.1-y Sn.sub.y Se Epitaxial Films, By J. D. Jensen and R. B. Schoolar, Journal of Vacuum Science Technology, Vol. 13, No. 4, (1976).
More recent efforts, although originating from a different direction, include:
Properties of PbS.sub.1-x Se.sub.x Epilayers Deposited Onto PbS Substrates By Hot-Wall Epitaxy, by K. Duh and H. Preier, Journal of Vacuum Science Technology, Vol. 10, pp. 1360, (1975).
PbSe Heteroepitaxy By The Hot-Wall Technique, by K. Duh and H. Preier, Thin Solid Films, Vol. 27, p. 247, (1975).
Double Heterojunction PbS-PbS.sub.1-x Se.sub.x -PbS Laser Diodes With CW Operation Up to 96K, by H. Preier, M. Bleicher, W. Riedl, and H. Maier, Applied Physics Letters, Vol. 28, No. 11, (June, 1976).
PbTe and Pb.sub.0.8 Sn.sub.0.2 Te Epitaxial Films On Cleaved BaF.sub.2 Substrates Prepared By A Modified Hot-Wall Technique, by T. Kasai, D. W. Bassett, and J. Hornung, Journal of Applied Physics, Vol. 47, (July, 1976).
Double-Heterostructure PbS-PbSe-PbS Lasers With CW Operation Up to 120K, by H. Preier, M. Bleicher, W. Riedl, and H. Maier, Journal of Applied Physics, Vol. 47, (December 1976); and,
Growth of PbTe Films Under Near-Equilibrium, by A. Lopez-Otero, Journal of Applied Physics, Vol. 48, No. 1, January 1977.
It is well established that single crystal films of PbS, PbSe, and related compounds, hereinafter referred to as lead salt alloys, can be epitaxially grown on heated alkali halide substrates by vacuum evaporation. It is also known that the conductivity type of these semiconductors in bulk form can be controlled by regulating deviation from stoichiometry. Anion (lead) vacancies make these crystals P-type and cation vacancies make them N-type.
In the past, films of the lead salts have been produced through the use of various deposition techniques. These films were homogeneous with constant energy gaps both across their length and throughout their thickness.